Biasing MA5002VZ

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   The service manual states that this should be done at 'room temperature' and the amp left to cool if it has been run heavily.  But should I do this immediately after turning on from cold or wait for the transistor temps to stabilize? (the bias current goes up over time).

Also is there a neat way to do this I currently have the top board flipped over with a bit of cardboard stopping it shorting on the other top board, I feel that the amp should have been designed in a better way.

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kipman725 Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 02 September 2020 Location: Warrington Status: Offline Points: 231	Post Options Post Reply Quote kipman725 Report Post Thanks(0) Quote Reply Topic: Biasing MA5002VZ Posted: 16 January 2021 at 11:43pm
	The service manual states that this should be done at 'room temperature' and the amp left to cool if it has been run heavily. But should I do this immediately after turning on from cold or wait for the transistor temps to stabilize? (the bias current goes up over time). Also is there a neat way to do this I currently have the top board flipped over with a bit of cardboard stopping it shorting on the other top board, I feel that the amp should have been designed in a better way.

kedwardsleisure Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4947	Post Options Post Reply Quote kedwardsleisure Report Post Thanks(0) Quote Reply Posted: 17 January 2021 at 9:33pm
	The bias setting automatically keeps it on track no matter what the temperature is although I always do it from cold. The ODEP voltages must be adjusted according to temperature, there's a table in the service manual so you read the heatsink temperature by monitoring the temperature sensor voltage, and then adjust the odep pots according to the table. This is the one that will alter as the heatsinks warm up. The pots for high and low side bias are adjustable without dismantling the board. You just remove the rear panel (the one that holds the PIP card in place) then loosen the 4 screws holding the 'main' control card plate and slide it up and back about 10mm. The screws run in slots either side the case. The pots are then visible, on one side they're facing you and the other they're accessible via holes in the pcb. Edited by kedwardsleisure - 17 January 2021 at 9:35pm
	Kevin North Staffordshire

kipman725 Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 02 September 2020 Location: Warrington Status: Offline Points: 231	Post Options Post Reply Quote kipman725 Report Post Thanks(0) Quote Reply Posted: 17 January 2021 at 11:46pm
	Many thanks ken I have figured out the moving the tray back trick. What I see on startup from cold for the bias is that there is a relatively small increase until the fans first turn on and then it stabilizes. Another issue has come up though R702 (blue disk thermistor) blew just after finishing setting the bias. I'm wondering if I caused this by power cycling about 10 times to set the bias (as I didn't want to turn the bias pot with the amp on and in such a precarious state). Is it common for these thermistors to fail or should I presume that there is a problem with the relay that should (presumably) be bypassing it?

kedwardsleisure Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4947	Post Options Post Reply Quote kedwardsleisure Report Post Thanks(0) Quote Reply Posted: 18 January 2021 at 12:32am
	you probably burned it out by switching it on and off too often.
	Kevin North Staffordshire